Small id api tidy and adding class to HTML if you are signed in

After chats with @stephanfowler and @phamann, I am going to be looking at making signed in elements visible / hidden dependant on classes.

This was the best situation between:
* Doing the logic on the server rendering on the same URL and breaking the cache. **Problem?** Breaks cache
* Having a separate URL for signed in content. **Problem?** Weird, and if calls aren't through XHR, creates template logic that breaks the cache.
* Creating content in the JS. **Problem?** It's getting very very messy already.

**What next?**
We'll be applying this logic to the reply buttons on discussion.